The present invention relates to an asynchronous PCM common decoding apparatus for decoding asynchronous PCM (pulse code modulation) signals sent from a plurality of transmitter sources.
In satellite communication, an FM (frequency modulation)/FDMA (frequency division multiple access) system is employed, in which speech signals are transmitted as frequency-division multiplexed signals after they have been used to FM-modulate a carrier. In place of performing such analog transmission, it is also possible to encode the speech signals into PCM signals at first and to transmit them as frequency division multiplexed signals after subjecting them to PSK (phase shift keying) modulation.
In the case where a PSK carrier wave transmitted from a given transmitting station comprises only speech signals (hereinafter referred to simply as signals) destined for one particular station, such PCM transmission according to FDMA systems can be realized in exactly the same manner as the conventional PCM transmission. However, in the case where there are signals destined for many stations and also the number of channels per one station is small, the multi-destination operation in which signals destined for many stations are multiplexed and comprised in one PSK carrier wave is more economical. Especially, in order to employ digital speech interpolation (DSI), it is essentially necessary that the number of channels is gathered to a certain extent, and in view of such aspects, the multidestinational operation becomes indispensable.
In FIG. 1 which shows a schematic block diagram of one example of the prior art systems, a communication system is operated on a multidestination basis among three stations represented by reference numerals 1, 2 and 3.
In a transmitter 10 of a station 1, input signals 100 are PCM encoded and time division multiplexed, and then, they are transmitted to stations 2 and 3. On the other hand, signals sent from the stations 2 and 3 are received and decoded in the station 1 by receivers 11 and 12, respectively. Among the signals received from the respective stations and decoded, extracted signals destined for its own station are output signals 101. Naturally, the number of the output signals 101 is equal to that of the input signals 100. Operations in the stations 2 and 3 are carried out similarly to the case of the station 1.
The reason why receiver units equal in number to the communicating stations are required in each station, is because the respective stations are operated by clock sources asynchronous to each other. Further, the reason why such asynchronous clock sources must be used, is because in an FDMA communication system there is not provided special equipment for synchronizing the clock sources in the respective stations, and as a result, the respective received signals are asynchronous to each other.
When the number of the communicating stations is large and the number of channels for each station is relatively small, by means of the above-described prior art systems, the economical advantages obtained by the multidestinational operation cannot be expected so much.
While one example of the above-described prior art system is found in an article by R. C. Davis and R. J. F. Fang entitled "CHANNEL CAPACITY EXTENSION VIA FDMA/PSK/DSI" p.p. 170-179 (especially page 172) and read at the "Third International Conference on Digital Satellite Communications " held on Nov. 11-13, 1975 in Kyoto, JAPAN, a detailed system construction has not been proposed therein.